Improved electrical properties of HfTiO/GeO x N y gate dielectric Ge MOS capacitors by using wet–NO Ge-surface pretreatment

Reactive cosputtering is employed to prepare high-permittivity HfTiO gate dielectric on n-Ge substrate. Effects of Ge-surface pretreatment on the interface and gate leakage properties of the dielectric are investigated. Excellent performances of Al/HfTiO/GeO _x N _y /n-Ge MOS capacitor with wet–NO surface pretreatment have been achieved with a interface-state density of 2.1×10¹¹ eV⁻¹ cm⁻², equivalent oxide charge of −7.67×10¹¹ cm⁻² and gate leakage current density of 4.97×10⁻⁵ A/cm² at V _g =1 V.     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.     

Influence of high energy electron irradiation on the characteristics of polysilicon thin film transistors

The influence of high energy electron (23 MeV) irradiation on the electrical characteristics of p-channel polysilicon thin film transistors (PSTFTs) was studied. The channel 220 nm thick LPCVD (low pressure chemical vapor deposition) deposited polysilicon layer was phosphorus doped by ion implantation. A 45 nm thick, thermally grown, SiO2 layer served as gate dielectric. A self-alignment technology for boron doping of the source and drain regions was used. 200 nm thick polysilicon film was deposited as a gate electrode. The obtained p-channel PSTFTs were irradiated with different high energy electron doses. Leakage currents through the gate oxide and transfer characteristics of the transistors were measured. A software model describing the field enhancement and the non-uniform current distribution at textured polysilicon/oxide interface was developed. In order to assess the irradiation-stimulated changes of gate oxide parameters the gate oxide tunneling conduction and transistor characteristics were studied. At MeV dose of 6×10¹³ el/cm², a negligible degradation of the transistor properties was found. A significant deterioration of the electrical properties of PSTFTs at MeV irradiation dose of 3×10¹⁴ el/cm² was observed.     

Direct fabrication and IV characterization of sub-surface conductive channels in diamond with MeV ion implantation

In the present work we report about the investigation of the conduction mechanism of sp² carbon micro-channels in single crystal diamond. The structures are fabricated with a technique which employs a MeV focused ion-beam to damage diamond in conjunction with variable thickness masks. This process changes significantly the structural properties of the target material, because the ion nuclear energy loss induces carbon conversion from sp³ to sp² state mainly at the end of range of the ions (few micrometers). Furthermore, placing a mask with increasing thickness on the sample it is possible to modulate the channels depth at their endpoints, allowing their electrical connection with the surface. A single-crystal HPHT diamond sample was implanted with 1.8 MeV He⁺ ions at room temperature, the implantation fluence was set in the range 2.1×10¹⁶-6.3×10¹⁷ ions cm^-2, determining the formation of micro-channels with a graded level of damage extending down to a depth of about 3 μm. After deposition of metallic contacts at the channels’ endpoints, the electrical characterization was performed measuring the I-V curves at variable temperatures in the 80-690 K range. The Variable Range Hopping model was used to fit the experimental data in the ohmic regime, allowing the estimation of characteristic parameters such as the density of localized states at the Fermi level. A value of 5.5×10¹⁷ states cm^-3 eV^-1 was obtained, in satisfactory agreement with values previously reported in literature. The power-law dependence between current and voltage is consistent with the space charge limited mechanism at moderate electric fields.     

Thin TiO2 grown by metal–organic chemical vapor deposition on (NH4)2S x -treated InP

The electrical characteristics of thin TiO₂ films prepared by metal–organic chemical vapor deposition grown on a p-type InP substrate were studied. For a TiO₂ film of 4.7 nm on InP without and with ammonium sulfide treatment, the leakage currents are 8.8×10⁻² and 1.1×10⁻⁴ A/cm² at +2 V bias and 1.6×10⁻¹ and 8.3×10⁻⁴ A/cm² at −2 V bias. The lower leakage currents of TiO₂ with ammonium sulfide treatment arise from the improvement of interface quality. The dielectric constant and effective oxide charge number density are 33 and 2.5×10¹³ cm², respectively. The lowest mid-gap interface state density is around 7.6×10¹¹ cm⁻² eV⁻¹. The equivalent oxide thickness is 0.52 nm. The breakdown electric field increases with decreasing thickness in the range of 2.5 to 7.6 nm and reaches 9.3 MV/cm at 2.5 nm.     

Ion synthesis of silver nanoparticles in viscous-fluid epoxy resin

A method is described for the ion synthesis of silver nanoparticles in epoxy resin that is in a viscousfluid state (viscosity 30 Pa s) during irradiation. The viscous-fluid or glassy polymer is implanted by 30-keV silver ions at a current density of 4 μA/cm² in the ion beam in the dose range 2.2 × 10¹⁶–7.5 × 10¹⁶ ions/cm². The epoxy layers thus synthesized contain silver nanoparticles, which are studied by transmission electron microscopy and optical absorption spectroscopy. The use of the viscous-fluid state increases the diffusion coefficient of the implanted impurity, which stimulates the nucleation and growth of nanoparticles at low implantation doses and allows a high factor of filling of the polymer with the metal to be achieved.     

3 MeV electron irradiation-induced defects in CuInSe2 thin films

3 MeV electron irradiation induced-defects in CuInSe₂ (CIS) thin films have been investigated. Both of the carrier concentration and Hall mobility were decreased as the electron fluence exceeded 1×10¹⁷ cm⁻². The carrier removal rate was estimated to be about 1 cm⁻¹. To evaluate electron irradiation-induced defect, the electrical properties of CIS thin films before and after irradiation were investigated between 80 and 300 K. From the temperature dependence of the carrier concentration in non-irradiated thin films, we obtained N_D=1.8×10¹⁷ cm⁻³, N_A=1.7×10¹⁶ cm⁻³ and E_D=18 meV from the SALS fitting to the experimental data on the basis of the charge balance equation. After irradiation, a new defect level was formed, and N_T0=1.4×10¹⁷ cm⁻³ and E_T=54 meV were also obtained from the same procedure. From the temperature dependence of Hall mobility, the ionized impurity density was discussed before and after the irradiation.     

Synthesis of Si nanoparticles with narrow size distribution by pulsed laser ablation

We synthesized Si nanoparticles by pulsed nanosecond-laser ablation. We applied a positive voltage bias during laser irradiation and effectively reduced size distribution. Scanning electron micrographs of samples showed the nanoparticles to be highly non-agglomerated. Si nanoparticles have the average diameter of 4–5 nm, the geometrical standard deviation of 1.35, and the density of 1.6 × 10¹²/cm². A MOS device showed excellent charge trap behavior with a flat-band voltage shift over 7 V, which can be applied for memory device applications.     

Direct patterning of gold oxide thin films by focused ion-beam irradiation

For direct writing of electrically conducting connections and areas into insulating gold oxide thin films a scanning Ar⁺ laser beam and a 30 keV Ga⁺ focused ion beam (FIB) have been used. The gold oxide films are prepared by magnetron sputtering under argon/oxygen plasma. The patterning of larger areas (dimension 10–100 μm) has been carried out with the laser beam by local heating of the selected area above the decomposition temperature of AuO_x (130–150 °C). For smaller dimensions (100 nm to 10 μm) the FIB irradiation could be used. With both complementary methods a reduction of the sheet resistance by 6–7 orders of magnitude has been achieved in the irradiated regions (e.g. with FIB irradiation from 1.5×10⁷ Ω/□ to approximately 6 Ω/□). The energy-dispersive X-ray analysis (EDX) show a considerably reduced oxygen content in the irradiated areas, and scanning electron microscopy (SEM), as well as atomic force microscopy (AFM) investigations, indicate that the FIB patterning in the low-dose region (10¹⁴ Ga⁺/cm²) is combined with a volume reduction, which is caused by oxygen escape rather than by sputtering. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 13 July 2000     

Electron irradiation induced defects in undoped and Te doped gallium phosphide

The aim of the present work is to investigate defects that are introduced to Gallium Phosphide (GaP) by electron irradiation as well as their dependence on the background doping. Undoped and Te doped n-type GaP have been irradiated with 1.5 MeV electrons at fluences of 5×10¹⁶ e/cm². Deep level transient spectroscopy assessment revealed the dependence of the trap characteristics on background doping.     

Effective activation energy of 16MeV proton irradiated QMG-YBCO

Ion irradiation effects on improvement of flux-pinning properties for quenched and melt growth YBCO samples (QMG-materials) were studied. Irradiation with 16MeV protons was performed up to a dose of 1×10¹⁶ p/cm². The effective activation energies of vortices are expressed with the formula, U_eff=g(T)h(H)f(J) ∝ (J/J_c)^−∝* [1-(T/T_x)²]^2/3*H^−1/3 at lower irradiation doses below 5×10¹⁵ p/cm². The J-dependence of U_eff changes from power-law to quesi-exponential dependence with an increment of irradiation dose.     

X-ray diffraction studies of silicon implanted with high-energy erbium ions

Silicon crystals after implantation of erbium ions with energies in the range 0.8–2.0 MeV and doses in the range 1×10¹²–1×10¹⁴ cm⁻² have been studied by two-and three-crystal x-ray diffraction. Three types of two-crystal reflection curves are observed. They correspond to different structural states of the implanted layers. At moderate doses (1×10¹²–1×10¹³ cm⁻²) a positive strain is observed, due to the formation of secondary radiation defects of interstitial type. An increase of the implantation dose is accompanied by the formation of an amorphous layer separating the bulk layer and a thin monocrystalline surface layer. At an implantation dose of 1×10¹⁴ cm⁻² the monocrystalline surface layer is completely amorphized. Parameters of the implantation layers are determined. A model of the transformation of structural damage is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 853–857 (May 1997)     

Defect formation in oxygen- and boron- implanted MOS structures after gamma irradiation

The effect of gamma irradiation on the interface states of ion-implanted MOS structures is studied by means of the thermally stimulated charge method. 10-keV oxygen- or boron- (O⁺ or B⁺) implanted samples are gamma-irradiated with ⁶⁰Co. Gamma irradiation creates electron levels at the SiSiO₂ interface of the samples in a different way depending on the type of the previously implanted atoms (O⁺ or B⁺). The results demonstrate that the concentration of the shallower levels (in the silicon band gap) of oxygen-implanted samples increases more effectively after gamma irradiation. The same irradiation conditions increase more intensively the concentration of the deeper levels (in the silicon band gap) of boron-implanted samples. Received: 17 June 2002 / Accepted: 31 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +359-2/975-3236, E-mail: kaschiev@issp.bas.bg     

Swift heavy ion beam mixing at V/Si interface

Vanadium silicides are of increasing interest because of applications in high temperature superconductivity and in microelectronics as contact materials due to their good electrical conductivity. In the present work ion beam induced mixing at Si/V/Si interface has been investigated using 120 MeV Au ions at 1 × 10¹³ to 1 × 10¹⁴ ions/cm² fluence at room temperature. V/Si interface was characterized by Grazing Incidence X-Ray Diffraction (GIXRD), Atomic Force Microscopy (AFM), Rutherford Backscattering Spectrometry (RBS) and Cross-sectional Transmission Electron Microscopy (XTEM) techniques before and after irradiation. It was found that the atomic mixing width increases with ion fluence. GIXRD and RBS investigations confirm the formation of V₆Si₅ silicide phase at the interface at the highest ion irradiation dose.     

Reduction of the annealing temperature of radiation-induced defects in ion-implanted MOS structures

Si-SiO₂ structures irradiated with 11-MeV electrons for 10 s and then implanted with B⁺ ions with an energy of 10 keV at a dose of 1.0×10¹² cm^-2 through the oxide were annealed at different temperatures. MOS capacitors including such oxide layers were studied by quasi-static C/V and thermally stimulated current (TSC) methods. A comparison of the radiation defect annealing of double-treated (electron-irradiated and ion-implanted) samples and of implanted-only samples was carried out. It is shown that a preceding low-dose high-energy electron irradiation of the samples leads to a lowering of the annealing temperature of radiation defects introduced by ion implantation. After annealing at 500 °C for 15 min, no TSC spectra for the double-treated samples were observed. The spectra of the other samples (which were not previously irradiated) showed that after the same thermal treatment only some of the radiation defects introduced by ion implantation are annealed. The difference between the annealed interface state density of previously electron-irradiated and current MOS structures is also demonstrated. A possible explanation of the results is proposed . PACS 61.82.Fk; 85.40.Ry; 61.80.Fe     

Relaxation of the conductivity of CsI-Tl after excitation by subnanosecond electron pulses

The pulsed conductivity is investigated for a CsI-Tl crystal having a Tl⁺ concentration N=8×10¹⁷cm⁻³ and excited by an electron beam (0.2 MeV, 50 ps, 10²–10⁴ A/cm ²). It is shown that the amplitude of the conduction current pulse is almost an order of magnitude lower than for “pure” CsI crystals irradiated under like conditions. The conduction current relaxation time is preserved up to τ=100 ps in this case. Under the experimental conditions, therefore, the lifetime of electrons in the conduction band is controlled by trapping at Tl⁺ centers. The electron capture cross section at a Tl⁺ center is determined: σ=7×10⁻¹⁶ cm², which agrees in order of magnitude with estimates of the capture cross section for a neutral trapping center. Fiz. Tverd. Tela (St. Petersburg) 40, 66–67 (January 1998)     

Enhanced radiation hardness of ZnO nanorods versus bulk layers

It is shown that ZnO nanorods grown by MOCVD exhibit enhanced radiation hardness against high energy heavy ion irradiation as compared to bulk layers. The decrease of the luminescence intensity induced by 130 MeV Xe⁺²³ irradiation at a dose of 1.5 × 10¹⁴ cm^–2 in ZnO nanorods is nearly identical to that induced by a dose of 6 × 10¹² cm^–2 in bulk layers. The change in the nature of electronic transitions responsible for luminescence occurs at an irradiation dose around 1 × 10¹⁴ cm^–2 and 5 × 10¹² cm^–2 in nanorods and bulk layers, respectively. High energy heavy ion irradiation followed by thermal annealing is also effective on the quality of ZnO nanorods grown by electrodeposition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.     

Plasma-based nitrogen tail pulse shutter for CO2-TEA lidar dial systems

This paper presents the construction, use and characterisation of a laser-induced sealed plasma shutter to clip off the nitrogen pulse tail of a CO₂-TEA laser-based lidar dial system. Investigation of the optimum gas filling pressure, temporal profile of the clipped pulse, and the laser threshold power intensities to achieve ionisation growth and breakdown in helium, argon, and nitrogen are also presented. Values of these power density thresholds lie between 3×10¹¹ W cm^-2–5×10¹² W cm^-2, 2×10¹¹ W cm^-2–2×10¹² W cm^-2 and 3×10¹³ W cm^-2–2×10¹⁴ W cm^-2 for helium, argon, and nitrogen, respectively. The range resolution attainable with the present clipped pulses is 15 m, which is 30 times better than that readily obtained with the nitrogen-tailed pulses. Field measurements of the lidar returns with the clipped pulse from a co-operative target are presented. Received: 27 December 1999 / Revised version: 11 February 2000 / Published online: 27 April 2000     

Investigation of H<Superscript>+</Superscript> implanted silicon wafers with two-beam cross-modulation photocarrier radiometry

Single beam laser-induced infrared photocarrier radiometry (PCR) has been applied for measuring transport properties of H⁺ ion-implanted silicon samples. The contrast between the PCR signals inside and outside the area of implantation was investigated for different doses and energies of implantation. The H⁺ ion-implantation range of doses and energies was 3×10¹⁴ cm^-2 - 3×10¹⁶ cm^-2 and 0.75 MeV–2 MeV, respectively. Furthermore, a two-beam cross-modulation PCR technique was introduced to perform the same type of measurements inside and outside the implanted area. Comparison between contrasts from single- and double-beam methods showed significantly higher degree of sensitivity for the two-beam PCR technique.